The invention relates to an NMR probe for the analysis of materials by nuclear magnetic resonance. The probe comprises a magnet, for example an electromagnet or a permanent magnet for generating a constant time-invariant polarization field Bo in the material to be examined and a radio frequency oscillation circuit for generating a pulsed high-frequency magnetic excitation field B1, which is superimposed on the polarization field Bo to generate echo signals S in the material which are determined by the NMR probe as material-characteristic measurement values. The echo signals can be measured on a time basis after changes of the magnetic field by way of two or more radio frequency pulses provided by the NMR probe, the third and subsequent pulses after an echo period tE. The signal to be measured is generated in the area around the probe in the magnetic field wherein the components of the two magnetic fields B0 and B1 are orthogonal to each other.
A mechanical NMR probe such as the probe of the NMR-MOUSE (Nuclear Magnetic Resonance—Mobile Universal Surface Explorer) is a mobile measuring apparatus by which nuclear magnetic resonance is used for the analysis of materials. With an NMR-MOUSE spatial material structures can be examined. Crystalline or glass-like materials as well as soft materials such as elastomers can be examined with respect to their molecular dynamics, and also liquids and biological materials can be analyzed, see for example G. Eidmann et al. “The NMR-MOUSE, a mobile universal surface explorer”, Journal of Magnetic Resonance, 1996, p. 104/109, and P Blümler et al., “Spatially resolved magnetic resonance”, Wiley-VCH-publishers, 1998, p. 195/209, or A. Guthausen et al. “NMR imaging and material research”, Chemie in unserer Zeit, 1998, page 73/84. The time-constant static magnetic polarization field B0 is generated with the NMR-MOUSE usually by means of one or several permanent magnets. The pulsed magnetic excitation field B1 is the magnetic component of a high frequency field, which is generated by a radio frequency coil, below called RF coil, as a component of an electric oscillation circuit, wherein the RF coil serves generally at the same time as the receiver coil for the echo signals S to be measured. For the polarization of the nuclear magnetization in the permanent magnetic polarization field B0 and for the generation and detection of the echo signals, spatially homogeneous magnetic fields are generally not needed. NMR-Mouse probes can therefore be small and inexpensive in comparison with the normal NMR apparatus. The form and size of the ambient volume which is utilized for nuclear magnetic resonance and from which the electromagnetic signals are detected, are defined on one hand by the orthogonal components of both magnetic fields B0 and B1, and, on the other hand, by the band-width of the radio frequency impulses and their time pattern. The profile of the magnetic fields can be changed by the dimensioning and the arrangement of the permanent magnets and the coil of the electrical radio frequency oscillation circuit.
DE 199 28 039 A1 discloses an NMR-MOUSE apparatus for the examination of flatware of polymer materials with embedded textiles wherein several NMR-MOUSE probes form a measuring plane for supporting the flatware (3). The flatware is scanned from the surface thereof; the penetration depth of the excitation fields depends on the dimensioning of the NMR-MOUSE probe. Herein, the spatial measuring area in the material to be examined is variable in three dimensions by displacement of the NMR-MOUSE, by varying the magnetic field by auxiliary coils and by changing the high-frequency field. In connection with the present NMR-MOUSE apparatus, it is however disadvantageous that the given inhomogeneities of the permanent magnetic polarization field B0 and the excitation field B1 result in a dissatisfactory yield of the measuring signals from the volume area excited by an NMR-MOUSE probe by means of the radio frequency circuit. The signal to noise ratio is insufficient for demanding requirements particularly if thin material layers have to be examined.
It is the object of the present invention to improve the signal-to-noise ratio for unilateral NMR probes, wherein the penetration depth of the measuring volume taking into consideration the respective thickness of the material to be analyzed is optimized.